The objective of this proposal is to investigate the toxicological implications of the fact that the polybrominated biphenyls (PBB*) readily undergo photolytic debromination to form a new array of congeners. The toxicity of halogenated biphenyls is dependent upon the number of halogens and position(s) of halogenation. Thus non-toxic PBB congeners can theoretically be readily converted to toxic congeners. The specific aims of the proposed research are to: 1. Chemically characterize the products of the photodecomposition of 2,4,5,2',4',5'-HexBB. The compounds will be purified, their structure determined by MS and NMR, and their physical and spectral properties established. 2. Assess the metabolism of the photolysis products. Metabolism would be considered possible if substrate disappeared in the presence of NADPH, O2 and microsomes. Tissue half-lives, which reflect in vivo metabolism, will also be conducted in conjunction with toxicity studies (see Specific Aim 3 below). 3. Assess the toxicity of the photolysis products. Toxicity will be assessed in rats by monitoring body weight gains, thymic atrophy and pathology, and liver pathology and microsomal enzyme induction. 4. Assess the significance of environmental photodecomposition. Samples taken from the environment of Michigan (farms and chemical dumps) will be subjected to GC analysis for the photodecomposition products identified under Specific Aim 1.